1. Field of the Invention
The invention relates to weldable aluminum alloys which can be worked into sheet form, and to processes for their production.
2. Discussion of the Background
The series 6000 alloys, as defined by the Aluminum Association nomenclature, were essentially developed in the form of shaped members, although some of those alloys such as 6061 or 6082 are generally produced in the form of plates or strips intended for stamping or pressing. Conventional series 6000 alloys are charged with magnesium to a level not far away from stoichiometry with respect to Mg.sub.2 Si.
Alloys which are included in the series 6000 family are described in French patent Nos. FR-A No. 2 375 332 and FR-A No. 2 360 684, corresponding to U.S. Pat. Nos. 4,174,232 and 4,082,578, respectively. Those alloys, while being less highly charged with magnesium than conventional alloys of the series 6000, have a much higher silicon content.
Patent application FR No. 2 375 332 describes a process in which a Si-rich alloy is treated in such a way as to produce a fine submicronic precipitation of Si (having a particle size of 0.1 to 0.5 .mu.m) under a condition of supersaturation. This size is intermediate between the eutectic phases present in the alloy and the hardening phases which are usually found in Al-Si-Mg-Cu alloys.
Although the above-indicated precipitation of Si is reported to afford a certain number of advantages, it also provides some disadvantages. Excessive silicon precipitates reduce the levels of capacity for shaping of the material. In addition, the presence of these silicon precipitates weakens the resistance of the alloy to corrosion under use.
French patent application No. 2 360 684 describes an Al-Si-Mg-Cu alloy containing at least one recrystallisation inhibitor element selected from the group Mn, Cr and Zr. However, the presence of these elements is not advantageous. Mn in particular involves a number of disadvantages:
Mn gives rise to solidification of the intermetallic compounds based on Fe, Mn and Si. These reduce the capacity for shaping of the alloy and can initiate a decohesion and rupture phenomena when shaping operations are carried out.
Mn increases the critical quenching speed and therefore limits options in heat treatments of thick products.
Mn imparts to the alloy a poor level of performance in regard to corrosion resistance.
And Mn is not suited to short-duration homogenization operations such as those which are generally carried out in tunnel furnaces.
The incorporation of Cr or Zr results in effects similar to those produced by Mn.
The problem with which one skilled in this art is faced is therefore the problem of producing a weldable Al-Si-Mg-Cu alloy which can be stamped or pressed, which does not suffer from the above-indicated disadvantages, and which has satisfactory mechanical properties in the hardened state. The Al-Si-Mg-Cu alloy must additionally possess good suitability for cold shaping in the quenched state, and good resistance to corrosion, as a result of a simple heat treatment which excludes the presence of any precipitation of submicronic phase essentially consisting of Si.
The art has not yet found a solution to these problems. There is therefore a strongly felt need for a weldable Al-Si-Mg-Cu alloy which does not suffer from the above disadvantages.